Known in recent years is a microorganism which accumulates a polymer of (R)-3-hydroxybutyric acid (hereinafter abbreviated as (R)-3-HB) within the cells thereof; that polymer corresponds to one of the moieties constituting the compound of the present invention. It is also known that there are a large number of microorganisms which accumulate within the cells thereof various kinds of (R)-3-hydroxyalkanoic acid units (hereinafter abbreviated as (R)-3-HA). (See P. A. Holmes, Phys. Technol., 1985, (16), p.32 and Seibunkaisei Kobunshi Zairyo (Biodegradable Polymeric Materials)., p.26, written by Yoshiharu Doi, published by Kogyo Chosa Kai, 1990.)
Further, a polymer having (R)-3-HB and a 4-hydroxybutyric acid unit (hereinafter abbreviated as 4-HB) had not been produced until it was synthesized by means of a microorganism (see polym. Commum., 29, 174 (1988)). Since these polymers have such properties as biodegradability or enzymatic degradability, hydrolyzability, and compatibility to the living body, they are attracting attention as a new type of functional materials (see Seibunkaisei Kobunshi Zairyo, p.19, written by Yoshiharu Doi, published by Kogyo Chosa Kai, 1990 and JP-A-4-292619). (The term "JP-A" as used herein means an unexamined published Japanese patent application".)
These microbial methods for polymer synthesis are disadvantageous in that since the methods utilize a microorganism or an enzymatic reaction, a troublesome step is needed, for example,to separate the polymer from the cells, and that the cost of production is high.
In addition, most of the polymers thus produced contain an (R)-3-HA unit and a 4-HB unit, and only one polymer has been reported which contains a 5-hydroxyvaleric acid unit (Makromol. Chem., Rapid Commun., 8, 631 (1987)).
On the other hand, the ring-opening polymerization of optically active .beta.-butyrolactone has been reported in Polymer Letters., 9, 173 (1970), Macromolecules, 23, 3206 (1990), and Macromolecules, 24, 5732 (1991). However, ring-opening copolymerization of the lactone with other lactone has been unknown so far.
With respect to the ring-opening copolymerization of racemic .beta.-butyrolactone with .delta.-valerolactone or .epsilon.-caprolactone, there is a report in J. Organomet. Chem., 341, p.83 (1988).
A low-molecular-weight polymer obtained by the condensation polymerization of racemic 3-HB with lactic acid has been reported in U.S. Pat. No. 3,579,549.
However, the production of a biodegradable polymer by the ring-opening polymerization of (R)-.beta.-butyrolactone has many problems from an industrial standpoint. Specifically, there are no simple methods for synthesizing optically active (R)-.beta.-butyrolactone, i.e., the monomer. Further, as compared with microbial polymerization methods, the ring-opening polymerization method is defective in that molecular weight is low and catalytic activity is also low, resulting in high production cost.